Back light unit for backlit displays

ABSTRACT

A back light unit includes an array of light emitting diodes, at least two optical films positioned above the array of light emitting diodes, and a pair of brightness enhancement films positioned above the at least two optical films. A majority of the optical films are light splitting optical films having a plurality of light splitting microstructures on at least one surface thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 62/898,693, filed Sep. 11, 2019, U.S.Provisional Patent Application Ser. No. 62/929,309, filed Nov. 1, 2019,and U.S. Provisional Patent Application Ser. No. 63/023,618, filed May12, 2020, the entire contents of all of which are incorporated herein byreference.

FIELD

The present invention is generally related to a back light unit of abacklit display, particularly for backlit displays with light emittingdiode (LED) light sources.

BACKGROUND

In the pursuit of improved image quality, liquid crystal displays (LCDs)are increasingly using a back light unit architecture 100, schematicallyillustrated in FIG. 1, that includes of an array 110 of individual shortwavelength (blue) LEDs 112. FIGS. 2A and 2B illustrate a typicalintensity distribution of light emitted from a single LED as a functionof angle, as measured by a goniophotometer. As illustrated, the LEDsource approximates a Lambertian source that emits a substantiallysymmetrical light distribution relative to the nadir, with the highestintensity of light at the nadir.

Returning to FIG. 1, a series of films may be used to spread or diffusethe light emitted from the blue LEDs 112 so that the back light unit 100may deliver a more uniform light to the LCD panel (not shown) containingthe liquid crystals located above the back light unit 100. Asillustrated, the back light unit 100 typically includes a diffuser film120, which may be a volumetric diffuser or a circular diffuser, a colorconversion layer 130 that uses either quantum dots or phosphor material,for example, to convert some of the blue light emitted by the LEDs 110to green and red light, a diffuser film 140, which may be a volumetricdiffuser or a circular diffuser resulting from a random texturedsurface, configured to spread or diffuse the light exiting the colorconversion layer 130, and two brightness enhancing films (BEFs) 150,160, which are often two prism films rotated approximately 90 degreesrelative to each other. There may be additional films in the back lightunit 100 that are used to improve the overall uniformity and brightnessof the light being delivered to the LCD panel. In some back light units,white LEDs may be used without a color conversion layer.

When LEDs 112 are arranged in an array, such as the array 110illustrated in FIG. 3, it is desirable to hide the individual LEDs 112and present a bright and uniform light to the LCD panel. As noted above,one approach to achieving this goal is to include one or more diffusers,such as the diffuser film 120, in the back light unit 100 to diffuse,spread, or blur the beams of light emitted by the LEDs 112. FIG. 4schematically illustrates such diffusion of the light emitted by asingle LED 112, with the darker shades of grey represent a brighterlight than the lighter shades of grey. Such diffusion may also reducethe mean energy of the light.

In addition, electronic devices that include LCDs are become thinner andthinner. As a result, the back light units of such displays are alsobecoming thinner and thinner, which presents another challenge to managethe light being emitted by the LEDs 112 in an effective manner. Forexample, when the diffuser film 120 is placed over the array 110 of LEDs112, as schematically illustrated in FIG. 5A, the individual points oflight emitted by the LEDs are diffused such that light having lessintensity from adjacent LED's 112 start to overlap to create areas oflight with higher intensity. If the thickness of the diffuser film 120is increased, which may be undesirable for thinner back light units 100,the individual points of light may be spread even further and providebetter uniformity of the light, but there are still brighter and darkerregions, as schematically illustrated in FIG. 5B.

It is desirable to have a back light unit 100 for an LCD display havingan array 110 of blue LEDs 112 and a thin profile, yet still deliverbright and uniform light to the LCD panel while effectively hiding theindividual LEDs 112.

SUMMARY

According to an embodiment of the invention, there is provided a backlight unit that includes an array of light emitting diodes, at least twooptical films positioned above the array of light emitting diodes, and apair of brightness enhancement films positioned above the at least twooptical films. A majority of the at least two optical films are lightsplitting optical films having a plurality of light splittingmicrostructures on at least one surface thereof.

In an embodiment, all of the at least two optical films have theplurality of light splitting microstructures on at least one surfacethereof.

In an embodiment, the back light unit includes a color conversion layerpositioned above the array of light emitting diodes and below the pairof brightness enhancement films. In an embodiment, the color conversionlayer is positioned above at least one light splitting optical film. Inan embodiment, the color conversion layer has at least one surfacecomprising a plurality of light splitting microstructures.

In an embodiment, the back light unit includes at least one additionallight splitting optical film positioned above the color conversion layerand below the pair of brightness enhancement films.

In an embodiment, the at least two optical films includes a first lightsplitting optical film comprising a plurality of first parallel linearprisms extending in a first direction on a first side thereof and aplurality of first elliptical lenticular structures extending in asecond direction on a second side thereof. The second direction issubstantially orthogonal to the first direction. The first side facesthe array of light emitting diodes. In an embodiment, the at least twooptical films include a second light splitting optical film positionedabove the first light splitting optical film. The second light splittingoptical film includes a plurality of second parallel linear prismsextending substantially in the first direction on a first side thereofand a plurality of second elliptical lenticular structures extending inthe second direction on a second side thereof. The first side of thesecond light splitting optical film faces the second side of the firstlight splitting optical film.

In an embodiment, the at least two optical films include a third lightsplitting optical film positioned above the second light splittingoptical film. The third light splitting optical film includes aplurality of third parallel linear prisms extending substantially in thesecond direction on a first side thereof. In an embodiment, the thirdlight splitting further includes a plurality of microstructures on asecond side thereof. In an embodiment, the second side of the thirdlight splitting optical film faces the second side of the second lightsplitting optical film.

In an embodiment, at least one of the optical films is a first lightsplitting optical film that includes a plurality of first parallellinear prisms extending in a first direction on a first side thereof anda plurality of second parallel linear prisms extending in the firstdirection on a second side thereof. In an embodiment, at least one ofthe optical films is a second light splitting optical film that includesa plurality of first parallel linear prisms extending in the firstdirection on a first side thereof and a plurality of second parallellinear prisms extending in the first direction on a second side thereof.In an embodiment, at least one of the optical films is a second lightsplitting optical film includes a plurality of first parallel linearprisms extending in a second direction, substantially orthogonal to thefirst direction, on a first side thereof and a plurality of secondparallel linear prisms extending in the second direction on a secondside thereof.

In an embodiment, at least one of the optical films is a first lightsplitting optical film that includes a plurality of first parallellinear prisms extending in a first direction on a first side thereof anda plurality of second parallel linear prisms extending in a seconddirection, substantially orthogonal to the first direction, on a secondside thereof.

In an embodiment, two of the optical films are light splitting opticalfilms. Each light splitting optical film includes a plurality ofmicrostructures on a first side thereof and a plurality of parallellinear prisms extending in a first direction on a second side thereof.Each microstructure has a shape of a quad pyramid.

In an embodiment, three of the optical films are light splitting opticalfilms. Each light splitting optical film includes a plurality ofmicrostructures on a first side thereof and a plurality of parallellinear prisms extending in a first direction on a second side thereof.Each microstructure has a shape of a quad pyramid.

According to an aspect of the invention, there is provided a back lightunit that includes an array of light emitting diodes, and a lower stackof optical films positioned above the array of light emitting diodes andconfigured to receive light emitted by the array of light emittingdiodes. The lower stack of optical films includes a first lightsplitting optical film that includes a plurality of first lightsplitting microstructures on a first side thereof facing the array oflight emitting diodes, the plurality of first light splittingmicrostructures constructed and arranged to split light received fromthe array of light emitting diodes. The lower stack of optical filmsincludes a second light splitting optical film positioned above thefirst light splitting optical film. The second light splitting opticalfilm includes a plurality of second light splitting microstructures on afirst side thereof facing the first light splitting optical film, theplurality of second light splitting microstructures constructed andarranged to split light received from the first light splitting opticalfilm. The back light unit includes a color conversion layer positionedabove the lower stack of optical films and configured to receive lightfrom the lower stack of optical films, an upper stack of optical filmspositioned above the color conversion layer and configured to receivelight from the color conversion layer, and a pair of brightnessenhancement films positioned above the upper stack of optical films andconfigured to receive light from the upper stack of optical films.

In an embodiment, the plurality of first light splitting microstructuresincludes a plurality of first parallel linear prisms, and the pluralityof second light splitting microstructures includes a plurality of secondparallel linear prisms oriented orthogonal to the plurality of firstparallel linear prisms.

In an embodiment, the first light splitting optical film also includes aplurality of first random rough microstructures on a second sidethereof, and the second light splitting optical film also includes aplurality of second random rough microstructures on a second sidethereof.

In an embodiment, the lower stack of optical films also includes a thirdoptical film positioned above the second light splitting optical film.In an embodiment, the third optical film includes a plurality ofmicrostructures facing the second light splitting optical film. In anembodiment, each of the plurality of microstructures of the thirdoptical film generally has the shape of a four-sided pyramid.

In an embodiment, the upper stack of optical films includes a thirdlight splitting optical film positioned above the color conversionlayer. In an embodiment, the upper stack of optical films also includesa fourth light splitting optical film positioned above the third lightsplitting optical film.

In an embodiment, the color conversion layer has at least one surfacethat includes a plurality of light splitting microstructures.

These and other aspects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification.It is to be expressly understood, however, that the drawings are for thepurpose of illustration and description only and are not intended as adefinition of the limits of the invention. As used in the specificationand in the claims, the singular form of “a”, “an”, and “the” includeplural referents unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

The components of the following figures are illustrated to emphasize thegeneral principles of the present disclosure and are not necessarilydrawn to scale, although at least one of the figures may be drawn toscale. Reference characters designating corresponding components arerepeated as necessary throughout the figures for the sake of consistencyand clarity.

FIG. 1 is a schematic illustration of a typical back light unit thatincludes an array of LEDs for an LCD display;

FIG. 2A is a three-dimensional plot of a distribution of light outputfrom an LED as a function of angle, as measured by a goniophotometer;

FIG. 2B is the measured light distribution of FIG. 2A represented in twodimensions;

FIG. 3 is a schematic illustration of a top view of a portion of thearray of LEDs of the back light unit of FIG. 1;

FIG. 4 is a schematic illustration of a top view of a distribution oflight output from a single LED after the light has passed through adiffuser film;

FIG. 5A is a schematic illustration of a top view of the array of LEDsof FIG. 3 after the light emitted by the LEDs has passed through thediffuser film;

FIG. 5B is a schematic illustration of the array of LEDs of FIG. 3 afterthe light emitted by the LEDs has passed through a diffuser film havinga thickness greater than the diffuser film used for FIG. 5A;

FIG. 6 is a schematic illustration of a back light unit for an LCDdisplay in accordance with embodiments of the invention;

FIG. 7 is a schematic illustration of a lower stack of optical films ofthe back light unit of FIG. 6 in accordance with embodiments of theinvention;

FIG. 8 is a schematic illustration of two light splitting optical filmsof the lower stack of optical films of FIG. 7 in accordance withembodiments of the invention;

FIG. 9 is a three-dimensional plot of a distribution of the light outputfrom the LED source having the light distribution of FIG. 2A after thelight has passed through the two light splitting optical films of FIG.8, as measured by the goniophotometer;

FIG. 10 is a three-dimensional plot of a distribution of the lightoutput from the LED source having the light distribution of FIG. 2Aafter the light has passed through the two light splitting optical filmsof FIG. 8 having a higher refractive index than the two light splittingoptical films having the light distribution of FIG. 9, as measured bythe goniophotometer;

FIG. 11 is a two-dimensional plot of the measured light distribution ofFIG. 10 and the measured distribution of light output from the LEDsource having the light distribution of FIG. 2A after the light haspassed through a circular diffuser;

FIG. 12A is a schematic illustration of a top view of a distribution oflight output from a single LED after the light has passed through thetwo light splitting optical films of FIG. 8;

FIG. 12B is a schematic illustration of a top view of a portion of thearray of LEDs of FIG. 6 after the light emitted by the LEDs has passedthrough the light splitting optical films of FIG. 8;

FIG. 13 is a three-dimensional plot of a distribution of the lightoutput from the LED source having the light distribution of FIG. 2Aafter the light has passed through the two light splitting optical filmshaving the light distribution of FIG. 10 and a circular diffuserproviding moderate diffusion, as measured by the goniophotometer;

FIG. 14 is a three-dimensional plot of a distribution of the lightoutput from the LED source having the light distribution of FIG. 2Aafter the light has passed through the two light splitting optical filmshaving the light distribution of FIG. 10 and a volumetric diffuserproviding very high diffusion, as measured by the goniophotometer;

FIG. 15 is a two-dimensional plot of the measured light distributions ofFIGS. 10, 13 and 14;

FIG. 16 is a schematic illustration of a third optical film of the lowerstack of optical films of FIG. 7 in accordance with embodiments of theinvention;

FIG. 17 is a three-dimensional plot of a distribution of the lightoutput from the LED source having the light distribution of FIG. 2Aafter the light has passed through the two light splitting optical filmshaving the light distribution of FIG. 10 and the third optical film ofFIG. 16, as measured by the goniophotometer;

FIG. 18 is an output plot from a modelling program that shows anintensity of light from an LED light source as a function of position intwo dimensions after the light has passed through two light splittingoptical films having a high refractive index and a volumetric diffuser;

FIG. 19 is an output plot from the modelling program that shows anintensity of light from an LED light source as a function of position intwo dimensions after the light has passed through two light splittingoptical films having a high refractive index and the third optical filmof FIG. 16; and

FIG. 20 is an output plot from the modelling program that shows anintensity of light from an LED light source as a function of position intwo dimensions after the light has passed through another embodiment oftwo light splitting optical films having a high refractive index and thethird optical film of FIG. 16;

FIG. 21A is a schematic illustration of a first side of a lightsplitting optical film in accordance with an embodiment of theinvention;

FIG. 21B is an enlarged photomicrograph of a portion of a second side ofthe light splitting optical film of FIG. 21A;

FIG. 22A a two-dimensional plot of a distribution of the light outputfrom the LED source having the light distribution of FIG. 2A after thelight has passed through a single light splitting optical filmillustrated in FIGS. 21A and 21B, as measured by a goniophotometer;

FIG. 22B a two-dimensional plot of a distribution of the light outputfrom the LED source having the light distribution of FIG. 2A after thelight has passed through a single light splitting optical film accordingto an embodiment of the invention, as measured by a goniophotometer;

FIG. 22C a two-dimensional plot of a distribution of the light outputfrom the LED source having the light distribution of FIG. 2A after thelight has passed through a single light splitting optical film accordingto an embodiment of the invention, as measured by a goniophotometer;

FIG. 23 is a schematic illustration of a light splitting optical film inaccordance with an embodiment of the invention;

FIG. 24 is a schematic illustration of a light splitting optical film inaccordance with an embodiment of the invention; and

FIG. 25 is a schematic illustration of a light splitting optical film inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 6 schematically illustrates a portion of a back light unit 600according to embodiments of the present invention. As illustrated, theback light unit 600 includes an array 610 of LEDs 612, which may be thesame blue light emitting LEDs 112 described above, a lower stack ofoptical films 620, a color conversion layer 630 above the lower stack ofoptical films 620, an upper stack of optical films 640, which mayinclude one or more diffuser films, above the color conversion layer630, a first brightness enhancement film (“BEF”) 650 above the upperstack of optical films 640, and a second brightness enhancement film(“BEF”) 660 above the first BEF 650. The first BEF 650 and the secondBEF 660 may have substantially the same structure, but turned 90°relative to each other, as is known in the art. The color conversionlayer 630 may include, for example, phosphor or quantum dots and beconfigured to change the wavelength of a portion of the light beingemitted from the LEDs 612, such as from a blue wavelength to red andgreen wavelengths, as is known in the art.

FIG. 7 is a more detailed schematic view of the lower stack of opticalfilms 620 of FIG. 6. As illustrated, the lower stack of optical films620, which are the optical films that are positioned between the LEDs612 and the color conversion layer 630, includes a first light splittingoptical film 622, a second light splitting optical film 624, and anoptional third optical film 626. The third optical film 626 may be, forexample, a volumetric diffuser film or another light splitting opticalfilm, as described in further detail below. Additional optical films maybe used in the lower stack of optical films 620. The illustratedembodiment is not intended to be limiting in any way.

As defined herein, an “optical film” is a polymeric film. As definedherein, a “light splitting optical film” is a polymeric film thatincludes a plurality of light splitting micro lenses or microstructureson at least one surface. As defined herein, a “light splittingmicrostructure” is a microstructure which when a collimated beam isdirected on axis to the microstructure, the collimated beam is splitinto two or more beams with a region of lower relative intensity onaxis.

For example, a light splitting microstructure may be in the form of aprism and split an incoming beam into two beams with the angle betweenthe two beams being dependent on the prism angle and refractive index ofthe prism material. In an embodiment, a prism with a 90-degree angle andrefractive index of 1.5 may split the incoming on-axis beam into twobeams at approximately ±25 degrees. In an embodiment, a light splittingmicrostructure may be in the form of a three-sided pyramid and split theincoming on-axis beam into three beams. In an embodiment, a lightsplitting microstructure may be in the form of a four-sided pyramid andsplit the incoming on-axis beam into four beams. In an embodiment, alight splitting microstructure may be in the form of a cone and splitthe incoming on-axis beam into a conical ring.

Such light splitting microstructures may be created using manytechniques known in the art. For example, in an embodiment, the shape ofthe light splitting microstructure may be cast onto a substrate using asuitable master mold, and thermally-curing polymer or ultraviolet (UV)light curing polymer, or the shape may be impressed into a thermoplasticsubstrate through compression molding or other molding, or may becreated at the same time as the substrate using extrusion-embossing orinjection molding. The microstructures may be produced by replicating amaster. For example, an optical film may be made by replication of amaster containing the desired shapes as described in U.S. Pat. No.7,190,387 B2 to Rinehart et al., entitled “Systems And Methods forFabricating Optical Microstructures Using a Cylindrical Platform and aRastered Radiation Beam”; U.S. Pat. No. 7,867,695 B2 to Freese et al.,entitled “Methods for Mastering Microstructures Through a SubstrateUsing Negative Photoresist”; and/or U.S. Pat. No. 7,192,692 B2 to Woodet al., entitled “Methods for Fabricating Microstructures by Imaging aRadiation Sensitive Layer Sandwiched Between Outer Layers”, assigned tothe assignee of the present invention, the disclosures of all of whichare incorporated herein by reference in their entirety as if set forthfully herein. The masters themselves may be fabricated using laserscanning techniques described in these patents and may also bereplicated to provide microstructures using replicating techniquesdescribed in these patents.

In an embodiment, laser holography, known in the art, may be used tocreate a holographic pattern that creates the desired microstructures ina photosensitive material. In an embodiment, projection or contactphotolithography, such as used in semiconductor, display, circuit board,and other common technologies known in the art, may be used to exposethe microstructures into a photosensitive material. In an embodiment,laser ablation, either using a mask or using a focused and modulatedlaser beam, may be used to create the microstructures including theindicia in a material. In an embodiment, micromachining (also known asdiamond machining), known in the art, may be used to create the desiredmicrostructures from a solid material. In an embodiment, additivemanufacturing (also known as 3D printing), known in the art, may be usedto create the desired microstructure in a solid material.

FIG. 8 schematically illustrates an embodiment of a first lightsplitting optical film 810 and a second light splitting optical film820, which may be used as the first light splitting optical film 622 andthe second light splitting optical film 624 of FIG. 7. The first lightsplitting optical film 810 is configured to receive individual beams oflight emitted by the array of LEDs 610 and split each beam of light intotwo beams of light. The second light splitting optical film 820 isconfigured to receive the beams of light from the first light splittingoptical film 810 and split each beam of light into two beams of light,thereby resulting in an individual beam of light received by the firstlight splitting optical film 810 being split into four beams of lightupon exiting the second light splitting optical film 820. As illustratedin FIG. 8, the first light splitting optical film 810 includes aplurality of light splitting microstructures 812 in the form of parallellinear prisms that extend across one side of the first light splittingoptical film 810 that faces downward (and toward the array of LEDs, notshown). The first light splitting optical film 810 also includes aplurality of random rough microstructures 814 on a side opposite theparallel linear prisms 812. Similarly, the second light splittingoptical film 820 includes a plurality of light splitting microstructures822 in the form of parallel linear prisms that extend across one side ofthe second light splitting optical film 820 that faces downward andtoward the first light splitting optical film 810, and a plurality ofrandom rough microstructures 824 on a side opposite the parallel linearprisms 822.

The first light splitting optical film 810 and the second lightsplitting optical film 820 are oriented relative to each other so thatthe plurality of light splitting microstructures 812 of the first lightsplitting optical film 810 is oriented 90° relative to the plurality oflight splitting microstructures 822 of the second light splittingoptical film 820, which allows the original beam of light from anindividual LED (see FIG. 2A) to be split into four beams of light, asmeasured by a goniophotometer and illustrated in FIG. 9. By increasingthe refractive index of the plurality of light splitting microstructures812, 822, the original beam of light from an individual LED may be splitinto four beams of light and spread even further, as measured by agoniophotometer and illustrated in FIG. 10.

FIG. 11 is a two-dimensional plot of the measured light distribution ofFIG. 10 (represented by 1100) and the measured distribution of lightoutput from the LED source having the light distribution of FIG. 2Aafter the light has passed through a circular diffuser (represented by1110). As illustrated, the pair of light splitting optical films 810,820 splits the light received from the LED and spreads the light wider(i.e., away from the nadir that is at 0°) than a circular diffuser, andalso suppresses the on-axis (i.e., nadir) light as compared to thecircular diffuser. The on-axis light is suppressed by reflecting theon-axis light back towards the LED, which helps hide the LED from beingseen above the pair of light splitting optical films 810, 820.

FIG. 12A schematically illustrates the effect of the two light splittingoptical films 810, 820 on the light emitted by a single LED 612, incontrast to the effect of a circular diffuser on the light emitted by asingle LED 112 schematically illustrated in FIG. 4. FIG. 12Bschematically illustrates the effect of the two light splitting opticalfilms 810, 820 on the light emitted by the array 610 of LEDs 612, incontrast to the effect of the circular diffuser on the array 110 of LEDs112 schematically illustrated in FIGS. 5A and 5B. As depicted, the lightoutput by the two light splitting optical films 810, 820 is generallybrighter and more uniform than the light output by the circulardiffusers.

To investigate a further enhancement of the uniformity of the lightoutput by the pair of light splitting optical films 810, 820, a circulardiffuser providing moderate diffusion was placed over the pair of lightsplitting optical films 810, 820 having the higher refractive index(output illustrated in FIG. 10), and the light passing through the stackof three films was measured with the goniophotometer. The result isillustrated in FIG. 13, and indicates that the Gaussian diffusion afterthe light was split in four by the pair of light splitting optical films810, 820 appears to suppress much of the desirable spreading of lightcreated by the pair of light splitting optical films 810, 820.

A volumetric diffuser providing very high diffusion was placed over thepair of light splitting optical films 810, 820 having the higherrefractive index, and the light passing through the stack of three filmswas measured with the goniophotometer. The result is illustrated in FIG.14, and indicates that the increased diffusion after the light was splitin four by the pair of light splitting optical films 810, 820 appears tofurther inhibit the desirable spreading of light created by the pair oflight splitting optical films 810, 820.

FIG. 15 is a two-dimensional plot of the measured light distributions ofFIGS. 10, 13 and 14. More specifically, FIG. 15 illustrates a comparisonof the two-dimensional light intensity distributions for light uponexiting the pair of light splitting optical films 810, 820 having thehigher refractive index (represented by 1100), upon exiting the circulardiffuser film providing moderate diffusion (represented by 1500) andupon exiting the volumetric diffuser providing very high diffusion(represented by 1510), and indicates that increasing diffusion decreasesthe desirable spreading provided by the pair of light splitting opticalfilms 810, 820.

Similar effects that were seen with the circular diffuser and volumetricdiffuser have been found with the color conversion layer 630 as well.Specifically, it has been found that phosphor films may also suppresssome of the desirable spreading of light created by two or more lightsplitting optical films 810, 820. Therefore, it may also be desirable touse the pair of light splitting optical films 810, 820 (and in someembodiments, a single light splitting optical film) above the colorconversion layer 630 in the upper stack of optical films 640 in additionto the lower stack of films 620 and/or to add light splittingmicrostructures to one or both surfaces of the color conversion layer630.

FIG. 16 schematically illustrates an optical film 1600, which may beused as the third optical film 626 in the lower stack of optical films620 in accordance with an embodiment of the invention. As illustrated,the optical film 1600 includes a plurality of microstructures 1610 inthe form of quad (four-sided) pyramids on one side thereof. The opticalfilm 1600 was placed on top of the pair of light splitting optical films810, 820 having the higher refractive index, with the plurality ofmicrostructures 1610 facing the pair of light splitting optical films810, 820, and the light passing through the stack of three films 810,820, 1600 was measured with the goniophotometer. The result isillustrated in FIG. 17 and indicates that after the light was split infour by the pair of light splitting optical films 810, 820, the opticalfilm 1600 having the plurality of microstructures 1610 in the form ofquad pyramids increases the uniformity of the light spreading providedby the light splitting optical films 810, 820 in both directions, whichis desirable. In an embodiment, the optical film 1600 having theplurality of microstructures 1610 in the form of quad pyramids may beused in place of the pair of light splitting optical films 810, 820.

In order to further investigate the effects of the stacks of opticalfilms in accordance with embodiments of the invention, LightToolsillumination design software by Synopsis, Inc. was used to model theeffects of various stacks of three optical films 622, 624, 626 in thelower stack of optical films 620 on the point spread function (“PSF”),which is the intensity of the light as a function of position (in x-ycoordinates) on top of the third optical film 626. FIG. 18 illustratesthe modeling result of using the pair of light splitting optical films810, 820 and a third film in the form of a volumetric diffuser providingvery high diffusion. Similar to what was measured with thegoniophotometer in FIG. 14, FIG. 18 illustrates a relatively narrowpoint spreading function (PSF).

FIG. 19 illustrates the results of using the pair of light splittingoptical films 810, 820 and the third optical film 1600 having theplurality of microstructures 1610. Similar to what was measured with thegoniophotometer in FIG. 17, FIG. 19 illustrates the maintenance of thehigh angular spreading by the plurality of microstructures 1610 (quadpyramids), as compared to the volumetric diffuser results of FIG. 18.

FIG. 20 illustrates the modeling results when using two crossed prismfilms having a high refractive index similar to the pair of lightsplitting optical films 810, 820 described above, but without theplurality of random rough structures 814, 824, and the third opticalfilm 1600 having the plurality of microstructures 1610. As definedherein, “high refractive index” means a refractive index of greater than1.65, such as 1.7, for example. As illustrated, the crossed film withprisms without the random rough microstructures on one side thereofprovide less uniformity than the pair of light splitting optical films810, 820 that have the random rough microstructures 814, 824, whichresults in four distinct spots (FIG. 20) as compared to a larger singlespot (FIG. 19).

EXAMPLES

In order to test the effects of different combinations of films in theback light unit 600, a series of combinations of optical films were usedfor the lower stack of optical films 620 and the upper stack of opticalfilms 640, with the same color conversion layer 630 (a phosphor film) inbetween the lower stack of optical films 620 and the upper stack ofoptical films 640. The films used for the lower stack of optical films620 and the upper stack of optical films 640 were a pair of lightsplitting optical films, each having a plurality of microstructures, anda pair of diffuser films in the form of volumetric diffusers. Fourdifferent combinations were used, as summarized in Table I below.

TABLE I SUMMARY OF STACKS OF OPTICAL FILMS-EXAMPLES 1-4 Lower Stack ofColor Conversion Upper Stack of Example Optical Films Layer OpticalFilms 1 2 volumetric phosphor film 2 volumetric diffusers diffusers 2 2crossed light phosphor film 2 volumetric splitting optical diffusersfilms with microstructures 3 2 volumetric phosphor film 2 crossed lightdiffusers splitting optical films with microstructures 4 2 crossed lightphosphor film 2 crossed light splitting optical splitting optical filmswith films with microstructures microstructures

Each Example 1-4 was placed on a light board that includes an array ofmini LEDs having a spacing of 1.6 mm. When the pair of crossed (i.e.,oriented 90° relative to each other) light splitting optical films wereused in the upper stack of optical films, the pair of crossed lightsplitting optical films as a unit were turned about 20° clockwiserelative to the array of mini LEDs. The total thickness of each stack,the relative mean energy emerging from the stack and the range/meanenergy of each stack were measured. The results are summarized in TableII below.

TABLE II SUMMARY OF TEST RESULTS-EXAMPLES 1-4 Total Thickness RelativeRange/Mean Energy Example (mm) Mean Energy (%) 1 0.969 38.9 6.3 2 0.89939.2 5.9 3 0.879 75.8 2.4 4 0.809 77.1 1.2

A higher relative mean energy indicates brighter light exiting the backlight unit 600, which is desirable, and a lower range/mean energyindicates more uniform light exiting the back light unit 600, which isalso desirable. The test results show that the back light units 600 thatincluded two crossed light splitting optical films in the upper stack ofoptical films 640 (Examples 3 and 4) had significantly greater meanenergy exiting the stacks and significantly lower range/mean energy ascompared to the back light units 600 that included two volumetricdiffusers in the upper stack (Examples 1 and 2). Example 4, which hadtwo crossed light splitting optical films in both the lower stack ofoptical films 620 and the upper stack of optical films 640 had thesmallest thickness, the highest mean energy and the lowest range/meanenergy, which is desirable.

Additional samples were made to investigate other combinations of filmsfor the lower stack of films 620 in the back light unit 600, as well asa different spacing for the array 610 of light emitting diodes 612. ForExample 5, a stack of three light splitting optical films was used forthe lower stack of optical films 620. A light splitting optical film2100 having the structures illustrated in FIGS. 21A and 21B and anoverall thickness of about 0.11 mm was used as the first light splittingoptical film 622. As illustrated, the light splitting optical film 2100includes a plurality of parallel linear prisms 2112 extending in a firstdirection FD on a first side 2110 of the light splitting optical film2100 (see FIG. 21A), and a plurality of elliptical lenticularmicrostructures 2122 having a 1° by 60° spread and extending in a seconddirection SD substantially orthogonal to the first direction FD wereprovided on a second side 2120 of the light splitting optical film 2100(see FIG. 21B). The prims 2112 were made from a material having arefractive index of about 1.7. For the second light splitting opticalfilm 624 for Example 5, the same light splitting optical film 2100 wasused, but with an overall thickness of about 0.2 mm. The plurality ofparallel linear prisms 2112 for each of the films were alignedsubstantially parallel to each other in the first direction FD, incontrast to the orientation illustrated in FIG. 8, and were oriented toface the array 610 of LEDs 612. FIG. 22A illustrates a two-dimensionalplot of a distribution of light output from the LED 612 having aLambertian distribution after the light has passed through the lightsplitting optical film 2100 of FIGS. 21A and 21B with the plurality ofparallel linear prisms facing the LED 612. The lighter color indicateshigher light intensity.

Example 5 also included a third light splitting optical film as thethird optical film 626, which included a plurality of randomized conicalmicrostructures on a first side facing the second light splittingoptical film 624 and a plurality of parallel linear prisms on a secondside of the third light splitting optical film 626, opposite the firstside. The prisms were made from a material having an refractive index of1.7 and the third light splitting optical film 626 had a thickness of0.2 mm.

For Example 6, four light splitting optical films were used for thelower stack of optical films 620. The first light splitting optical film622 for this embodiment had a plurality of linear prisms on a bottomside facing the array 610 of LEDs 612 and a plurality of circular lightsplitting microstructures on a top side of the first light splittingoptical film 622. The first light splitting optical film 622 for thisembodiment had a thickness of 0.17 mm and the prisms were made from amaterial having a refractive index of about 1.7. FIG. 22B illustrates atwo-dimensional plot of a distribution of light output from the LED 612having a Lambertian distribution after the light has passed through thefirst light splitting optical film of this embodiment, with theplurality of parallel linear prisms facing the LED 612. The lightercolor indicates higher light intensity.

The second light splitting optical film 624 for this embodiment had aplurality of parallel linear prisms on a bottom side facing the array610 of LEDs 612 and a plurality of randomized conical microstructures ona top side of the second light splitting optical film 624. The secondlight splitting optical film 624 for this embodiment had a thickness of0.12 mm and the prisms were made from a material having a refractiveindex of about 1.7. FIG. 22C illustrates a two-dimensional plot of adistribution of light output from the LED 612 having a Lambertiandistribution after the light has passed through the second lightsplitting optical film of this embodiment, with the plurality ofparallel linear prisms facing the LED 612. The lighter color indicateshigher light intensity.

The second light splitting optical film 624 was oriented relative to thefirst light splitting optical film 622 such that the plurality ofparallel linear prisms of the second light splitting optical film 624were substantially orthogonal to the plurality of parallel linear prismsof the first light splitting optical film 622, similar to what isillustrated in FIG. 8.

The third light splitting optical film 626 for this Example 6 embodimenthad a plurality of circular light splitting microstructures on a bottomside facing the second light splitting optical film 624 and a pluralityof parallel linear prisms on a top side of the third light splittingoptical film 626. The film had a thickness of 0.11 mm and the prismswere made from a material having a refractive index of about 1.7. Theplurality of parallel linear prisms of the third light splitting opticalfilm were oriented to be parallel to the plurality of parallel linearprisms of the second light splitting optical film 624. The fourth lightsplitting optical film was the same as the third light splitting opticalfilm 626, but with the plurality of parallel linear prisms orientedsubstantially orthogonal to the plurality of parallel linear prisms ofthe third light optical splitting film 626.

Also included in Examples 5 and 6 were a phosphor film having athickness of 0.12 mm that was used for the color conversion layer 630and located above the third light splitting optical film 626, and a pairof crossed brightness enhancement films 650, 660, each having athickness of 0.1 mm, located above the color conversion layer 630. Noupper stack of optical films 640 was used between the color conversionlayer 630 and the pair of brightness enhancement films 650, 660. Asummary of the light splitting optical films that were used for Examples5 and 6 are summarized in Table III below.

TABLE III SUMMARY OF LOWER STACKS OF OPTICAL FILMS-EXAMPLES 5 & 6 FirstLight Second Light Third Light Fourth Light Exam- Splitting SplittingSplitting Splitting ple Optical Film Optical Film Optical Film OpticalFilm 5 Elliptical Elliptical Parallel None Lenticular Lenticular LinearPrisms Structures (top) Structures (top) (top) Parallel ParallelRandomized Linear Prisms Linear Prisms Conical (bottom) (bottom)Microstructures (bottom) 6 Circular Light Randomized Parallel ParallelSplitting Conical Linear Prisms Linear Prisms MicrostructuresMicrostructures (top) (top) (top) (top) Circular Light Circular LightParallel Parallel Splitting Splitting Linear Prisms Linear Prisms Micro-Micro- (bottom) (bottom) structures structures (bottom) (bottom)

Each of Examples 5 and 6 was placed on a light board that includes anarray of mini LEDs having a spacing of 2.4 mm. The total thickness ofeach stack (including the color conversion layer and brightnessenhancement films), the relative mean energy emerging from the stack andthe range/mean energy of each stack were measured. The results aresummarized in Table IV below.

TABLE IV SUMMARY OF TEST RESULTS-EXAMPLES 5 & 6 Total Thickness RelativeRange/Mean Energy Example (mm) Mean Energy (%) 5 0.83 69.6 2.3 6 0.8368.9 3.6

The test results for Examples 5 and 6 show that the back light unit 600that included three light splitting optical films in the lower stack ofoptical films 620 (Examples 5) had greater mean energy exiting the stack(greater brightness) and lower range/mean energy (greater uniformity) ascompared to the back light unit 600 that included four light splittingoptical films in the lower stack of optical films 620 (Example 6), eventhough the two lower stacks of optical films has the same thickness.

The test results indicate that it may be advantageous to use two or morelight splitting optical films in the lower stack of optical films 620having elliptical lenticular structures on top surfaces thereof andparallel linear prisms on bottom surfaces thereof, with the parallellinear prisms for the two films oriented substantially in the samedirection, i.e. within 30 degrees or desirably within 15 degrees.Although the elliptical lenticular structures described above had a 1°by 60° spread, other shapes may be used. For example, according toembodiments of the invention, elliptical lenticular structures havingspread of 1° by 40° or 1° by 90° may be used.

FIG. 23 schematically illustrates an embodiment of a light splittingoptical film 2300 that may be used as one or more of the light splittingoptical films 622, 624 in the lower stack of optical films 620illustrated in FIGS. 6 and 7. As illustrated, the light splittingoptical film 2300 includes a plurality of parallel linear prisms 2312extending in a first direction FD on a first side 2310 of the lightsplitting optical film 2300, and a plurality of parallel linear prisms2322 also extending in the first direction FD on a second side 2320 ofthe light splitting optical film 2300. In an embodiment, when two of thelight splitting optical films 2300 are used as the first and secondlight splitting optical films 622, 624 of the lower stack of opticalfilms 620, all of the parallel linear prisms 2312, 2322 of both films2300 may be aligned in substantially the same direction, e.g., the firstdirection FD.

In an embodiment, when two of the light splitting optical films 2300 areused as the first and second light splitting optical films 622, 624 ofthe lower stack of optical films 620, one of the two light splittingoptical films 2300 may be oriented so that the plurality of linearprisms 2312, 2322 of one film are aligned substantially orthogonal tothe plurality of linear prisms 2312, 2322 of the other film. Forexample, one film 2300 may have its plurality of linear prisms 2312,2322 aligned in the first direction FD, while the other film has itsplurality of linear prisms 2312, 2322 aligned in a second direction SD,substantially orthogonal to the first direction FD.

In an embodiment, when two of the light splitting optical films 2300 areused as the first and second light splitting optical films 622, 624 ofthe lower stack of optical films 620, one of the two light splittingoptical films 2300 may be oriented so that its plurality of linearprisms 2312, 2322 are aligned in the first direction FD, while the otherfilm has its plurality of linear prisms 2312, 2322 aligned in anydirection relative to the first direction FD, e.g., in a directionbetween the first direction FD and the second direction SD.

FIG. 24 schematically illustrates an embodiment of a light splittingoptical film 2400 that may be used as one or more of the light splittingoptical films 622, 624 in the lower stack of optical films 620illustrated in FIGS. 6 and 7. As illustrated, the light splittingoptical film 2400 includes a plurality of parallel linear prisms 2412extending in a first direction FD on a first side 2410 of the lightsplitting optical film 2400, and a plurality of parallel linear prisms2422 extending in a second direction SD, substantially orthogonal to thefirst direction FD, on a second side 2420 of the light splitting opticalfilm 2400. In an embodiment, when two of the light splitting opticalfilms 2400 are used as the first and second light splitting opticalfilms 622, 624 of the lower stack of optical films 620, all of theparallel linear prisms 2412 of the first sides 2410 of the films 2400may be aligned in substantially the same direction, e.g., the firstdirection FD.

In an embodiment, when two of the light splitting optical films 2400 areused as the first and second light splitting optical films 622, 624 ofthe lower stack of optical films 620, one of the two light splittingoptical films 2400 may be oriented so that the plurality of linearprisms 2412 of its first side 2410 are aligned substantially orthogonalto the plurality of linear prisms 2412 of the first side 2410 of theother film 2400 such that one film has its plurality of linear prisms2412 aligned in the first direction FD, while the other film has itsplurality of linear prisms 2412 aligned in the second direction SD,substantially orthogonal to the first direction FD.

In an embodiment, when two of the light splitting optical films 2400 areused as the first and second light splitting optical films 622, 624 ofthe lower stack of optical films 620, one of the two light splittingoptical films 2400 may be oriented so that its plurality of linearprisms 2412 of the first side 2410 are aligned in the first directionFD, while the other film has its plurality of linear prisms 2412 of thefirst side 2410 aligned in any direction relative to the firstdirection, e.g., in a direction between the first direction FD and thesecond direction SD.

FIG. 25 schematically illustrates an embodiment of a light splittingoptical film 2500 that may be used as one or more of the light splittingoptical films 622, 624 in the lower stack of optical films 620illustrated in FIGS. 6 and 7. As illustrated, the light splittingoptical film 2500 includes the plurality of quad (four-sided) pyramids1610 described above with respect to FIG. 16 on a first side 2510 of thelight splitting optical film 2500, and a plurality of parallel linearprisms 2522 extending in a first direction FD on a second side 2520 ofthe light splitting optical film 2500. In an embodiment, when two of thelight splitting optical films 2500 are used as the first and secondlight splitting optical films 622, 624 of the lower stack of opticalfilms 620, all of the parallel linear prisms 2522 of the second sides2520 of the films 2500 may be aligned in substantially the samedirection, e.g., the first direction FD.

In an embodiment, when two of the light splitting optical films 2500 areused as the first and second light splitting optical films 622, 624 ofthe lower stack of optical films 620, one of the two light splittingoptical films 2500 may be oriented so that the plurality of linearprisms 2522 of its second side 2520 are aligned substantially orthogonalto the plurality of linear prisms 2522 of the second side 2520 of theother film 2500 such that one film has its plurality of linear prisms2522 aligned in the first direction FD, while the other film has itsplurality of linear prisms 2522 aligned in the second direction SD,substantially orthogonal to the first direction FD.

In an embodiment, the third optical film 626 of the lower stack ofoptical films 620 of FIGS. 6 and 7 may also be the light splittingoptical film 2500 of FIG. 25, with the plurality of linear prisms 2522aligned in the first direction FD or the second direction SD.

The embodiments described herein represent a number of possibleimplementations and examples and are not intended to necessarily limitthe present disclosure to any specific embodiments. Instead, variousmodifications can be made to these embodiments, and differentcombinations of various embodiments described herein may be used as partof the invention, even if not expressly described, as would beunderstood by one of ordinary skill in the art. For example, the lightsplitting optical films and the diffuser optical films may includedifferent microstructures and different combinations of microstructuresthan the microstructures depicted in the drawings, such as, for example,the microstructures disclosed in International Patent ApplicationPublication No. WO 2019/152382, the entire content of which isincorporated herein.

In addition, the upper stack of optical films 640 may include the samecombination of films as the lower stack of optical films 620 or mayinclude a different combination of films. In an embodiment, the majorityof the films of the back light unit 600 that are located below thebrightness enhancement films 650, 660 may have microstructuresconfigured to split an incoming beam of light into two or more beams oflight. In an embodiment, all or almost all of the optical films in theback light unit 600 may have microstructures configured to split anincoming beam of light into two or more beams of light on at least onesurface thereof. The resulting brightness and uniformity of the lightexiting the lower stack of optical films 620 may be adjusted by usingdifferent combinations of prisms and microstructures on the variousoptical films in the lower stack of optical films 620.

The illustrated and above-described embodiments are not intended to belimiting in any way, and any such modifications to the embodimentsdescribed herein are intended to be included within the spirit and scopeof the present disclosure and protected by the claims that follow.

1-26. (canceled)
 27. A back light unit comprising: an array of lightemitting diodes; at least two optical films positioned above the arrayof light emitting diodes; and at least one brightness enhancement filmpositioned above the at least two optical films; wherein a majority ofthe at least two optical films are light splitting optical films havinga plurality of light splitting microstructures on at least one surfacethereof and at least one of the light splitting microstructures beingformed in a shape of a pyramid.
 28. The back light unit according toclaim 28, wherein the at least one of the light splittingmicrostructures is formed in a shape of a four-sided pyramid.
 29. Theback light unit according to claim 28, wherein the at least one of thelight splitting microstructures is formed in a shape of a three-sidedpyramid.
 30. The back light unit according to claim 28, wherein the atleast one of the light splitting microstructures is formed in a shape ofan inverted pyramid.
 31. The back light unit according to claim 28,wherein all of the at least two optical films have the plurality oflight splitting microstructures on at least one surface thereof.
 32. Theback light unit according to claim 28, further comprising a colorconversion layer positioned above the array of light emitting diodes andbelow the at least one brightness enhancement film
 33. The back lightunit according to claim 33, wherein the color conversion layer ispositioned above at least one light splitting optical film.
 34. The backlight unit according to claim 33, wherein the color conversion layer hasat least one surface comprising a plurality of light splittingmicrostructures.
 35. The back light unit according to claim 33, furthercomprising at least one additional light splitting optical filmpositioned above the color conversion layer and below at leastbrightness enhancement film.
 36. The back light unit according to claim28, wherein the at least two optical films include a third lightsplitting optical film comprising a plurality of parallel linear prismson a first side thereof.
 37. The back light unit according to claim 36,wherein the third light splitting further comprises a plurality ofmicrostructures on a second side thereof.
 38. The back light unitaccording to claim 37, wherein the second side of the third lightsplitting optical film faces the second light splitting optical film.39. The back light unit according to claim 36, wherein the third lightsplitting film comprises a plurality of microstructures on a first sidefacing the second light splitting optical film and a plurality of linearprisms on a second side of the third light splitting optical film,opposite the first side.
 40. The back light unit according to claim 28,wherein at least one of the at least two optical films comprise filmmicrostructures facing away from the light emitting diode.
 41. The backlight unit according to claim 28, wherein at least one of the at leasttwo optical films comprise a plurality of microstructures on a firstside facing a plurality of microstructures on the other one of the atleast two optical films.
 42. The back light unit according to claim 28,wherein at least one of the least two optical films comprises aplurality of first parallel linear prisms extending in the firstdirection on a first side thereof and a plurality of second parallellinear prisms extending in the first direction on a second side thereof.43. The back light unit according to claim 42, wherein at least one ofthe at least two optical films comprise a plurality of first parallellinear prisms extending in a second direction, substantially orthogonalto the first direction, on a first side thereof and a plurality ofsecond parallel linear prisms on a second side thereof.
 44. The backlight unit according to claim 28, wherein the at least two optical filmscomprise three light splitting optical films, each light splittingoptical film comprising a plurality of microstructures on a first sidethereof and a plurality of parallel linear prisms extending in a firstdirection on a second side thereof, wherein each microstructure has ashape of a quad pyramid.
 45. The back light unit according to claim 28,wherein the at least one brightness enhancement film positioned abovethe at least two optical films comprise a pair of brightness enhancementfilms.
 46. A back light unit comprising: an array of light emittingdiodes; at least two optical films positioned above the array of lightemitting diodes; and at least one brightness enhancement film positionedabove the at least two optical films; wherein the at least two opticalfilms comprise a first light splitting optical film comprising aplurality of first parallel linear prisms extending in a first directionon a first side thereof and a plurality of first elliptical lenticularstructures extending in a second direction on a second side thereof, thesecond direction being substantially orthogonal to the first direction,wherein the first side faces the array of light emitting diodes.
 47. Aback light unit comprising: an array of light emitting diodes; at leasttwo optical films positioned above the array of light emitting diodes;and at least one brightness enhancement film positioned above the atleast two optical films; wherein the at least two optical films includesa first light splitting optical film and a second light splittingoptical film that is positioned above the first light splitting opticalfilm, the second light splitting optical film comprising a plurality ofparallel linear prisms extending substantially in a first direction on afirst side thereof and a plurality of elliptical lenticular structuresextending in the second direction on a second side thereof.
 48. A backlight unit comprising: an array of light emitting diodes; at least twooptical films positioned above the array of light emitting diodes; andat least one brightness enhancement film positioned above the at leasttwo optical films; wherein at least one of the at least two opticalfilms is a first light splitting optical film comprising a plurality offirst parallel linear prisms extending in a first direction on a firstside thereof and a plurality of second parallel linear prisms extendingin a second direction, substantially orthogonal to the first direction,on a second side thereof.